The strakes were fitted as a result of some early spinning accidents soo after the Chippy entered service. At the time, the standard spin entry was done from a stall and there was a standard spin recovery used on all types. This was less than 100% successful on the Chippy and as a result the stakes were fitted (Similar strakes were fitted to Tiger Moths). These act as spoilers at high angles of attack and re-energise the airflow and spread it out a bit which made the rudder more effective thus aiding recovery. At about the same time, the wide chord rudder was fitted. But this was to aid control during aerobatics and not for spin recovery.
Also, the entry and more importantly, the recovery was modified. The entry was now not done from the stall, but from a reinforced entry made using full pro spin controls from 50 knots. The new recovery introduced a pause after the application of rudder - the usual technique was to count to two bananas. In addition, it was noted that there could be a null point in the elevator feel which made moving the stick forward more difficult past neutral. This is why there is a placard in civilian Chipmunks which mentions that the column may have to be moved all the way forwards.
This entry and recovery works fine and in about a thousand spins in the Chippy using these, I have never had an issue. I have also entered spins from the stall, and these are very different. They are flatter, slower and take longer to recover. I suspect it is in this spin that the strakes are designed for and recovering a strakeless aircraft in this event would lead to using far more heartbeats. I have spoken to pilots who have spun strakeless Chippys and they say there is no discernible difference - but I suspect they were using the 'new' techniques.
Spinning and aerobatics are prohibited on the strakeless aircraft in the UK and Australia - and I think NZ as well.